Thanks Chemie
I just went out at lunch time and bought a Du bro 16oz tank - the dimension from the back of the firewall to the inside of the former is 5 15/16" (151mm) the 16oz du bro tank is 5 7/8" (149mm). Should just squeek in - might have to remove the servo rail to do it though[

].
Have you had a chance to maiden your extra yet?